The present invention relates to an apparatus for the coating of a travelling web, said apparatus comprising means for the supply of coating agent to the web.
The basic principle in the coating of travelling webs, in particular paper webs, is that coating agent or coating composition is supplied to the web in excess. This excess of coating agent is then drained and recirculated. A typical example are so called blade coaters where the excess is scraped off by means of a flexible blade in engagement with the web. The pressure of the blade against the web is counterbalanced by an equally large reaction pressure which arises in two different ways, namely:
(a) from the web, i.e. the blade engages the highest points of the web which are partly compressed by the blade pressure, and
(b) from the hydrodynamic pressure formed in the pool of coating composition which is present before the entrance to the blade nip.
The pressure depends on web speed, the viscosity of the coating agent, the angle of the blade to the web and the applied quantity of coating agent. In practical operation usually a combination of the reaction presssures given under (a) and (b) is at hand. In those cases wherein factor (a) is predominant, i.e. the blade is supported against the web so that coating agent will fill up only the valleys and cavities of the web, there usually will be obtained an even coating profile along and across the web.
In those cases where factor (b) is predominant (for example with paper qualities having high surface eveness and the coating weight is high) the blade will mainly "flow" on a film of coating agent. In this case the pressure of the blade will be mainly balanced by the hydrodynamic pressure which, as previously indicated, is dependent on the factors:
(c) web speed PA1 (d) viscosity PA1 (e) blade angle PA1 (f) the quantity of coating composition transported up to the blade, i.e. the quantity of coating composition applied to the web. PA1 Q=dry coating weight (g/m.sup.2) PA1 n=excess from applicator (times) PA1 p=density of coating liquid (g/cm.sup.3) PA1 p=coating liquid solid's contents (%)
The factors according to items (c) to (e) can with sufficient accuracy be maintained constant using techniques available today. However, factor (f) is dependent on the ability of the applicator to deposit on the web layer in the correct quantity along and across the web.
In blade coating it is conventional techniques to deposit coating agent on the web using so called coating rolls or fountain applicators. Thus, an even layer of coating agent is transferred onto the web by the applicator. The thickness of the layer can be calculated according to the following formula: ##EQU1## where t=thickness of layer from applicator (mm)
As examples can be given ##EQU2##
In a conventional applicator the layer is deposited by passing the liquid through a slit which is formed between the web supported by a backing roll and the exit rib of the fountain or the roll surface in roll applicators. In so called jet-fountain coaters the fountain has a larger distance to the web and the liquid is supplied to the web through a narrow slit. In the so called twin-blade techniques the web is brought to engagement against the exit slit by deflecting the web across the slit.
The difficulty in coating a web with an even layer having a thickness of the order as given in the example above in machines of widths used in paper manufacture has, in practice, been found to be quite pronounced. These difficulties may for example be the following:
include varying slit dimensions between paper web and exit slit in applicators or rolls depending on manufacture tolerances, heat tensions, deflexions, etc. In jet-fountain coaters the difficulties can reside in for example clogging of the slit, tolerance of the slit opening and the pressure distribution in the extruder. In fountain applicators according to the so called twin-blade techniques, obtaining even pressure distribution in the exit slit and maintaining even tension along and across the web.
Another type of coaters is the so called Short-dwell-bladecoater where the liquid is pumped into a chamber the front wall of which is formed by the coating blade. The excess of coating agent usually is drained out in the slit between the rear wall and the web supported by the backing roll. It is true that with this coater an even layer must not be deposited before the blade whereby some of the difficulties previously described are reduced, but the difficult problem of even pressure distribution in the chamber remains. Another problem which is associated with these techniques is that enclosed air bubbles do not disappear before the blade but pass the blade and result in so called skip-coating.